Same here. I used to use them, until around 8-9 years ago. I realised they were using my car to shuttle people between the airport and their depot. I only noticed this since we rarely used the rear seats, but when I got it back all the doors were unlocked (it was an old car where you had to push the button down to lock each door).

Then another time I realised they had been having a good rummage around (no doubt looking for anything worth nicking!). I only realised this because the glove box latch was broken but it was firmly shut - someone had clearly given it a good yank and cracked the entire frame of the glove box. Needless to say that was the last time I used any of these firms. I'm sure some are better than others, but some of the drivers they use look pretty dodgy.

If it's just for a few days I always go for the short-stay pre-booked deals.